1. Field of the Invention
The present invention relates, in general, to an apparatus and method for film deposition, and, more particularly, to an apparatus and method for filling high aspect ratio gaps and vias with collimated particulate streams.
2. Statement of the Problem
Integrated circuit technology has advanced through continuing improvements in photolithographic processing so that smaller and smaller features can be patterned onto the surface of a substrate. Spaces or gaps exist between these patterned features. Integrated circuit surfaces also contain trench or via structures extending down into the surface. The lateral dimension of such structures is hereafter referred to as the width of the gap, trench or via; the vertical dimension of such structures is referred to as the depth. The aspect ratio is the ratio of depth to width.
The smaller features, with smaller spaces between features, result in high aspect ratio gaps, trenches and vias. These high aspect ratio structures must be filled with an appropriate material before continued processing. Deposition onto trench and via structures is commonly practiced at several stages in the fabrication of semiconductor devices and interconnections. Most often the objective is to provide highly conformal films or void-free (and preferably seam-free) filling.
High aspect ratio structures are difficult to fill due to xe2x80x9cshadowingxe2x80x9d effects wherein some particles traveling in random or uncontrolled directions (hereinafter xe2x80x9cisotropicxe2x80x9d particles) strike the sidewalls of the via at an angle thus causing a film growth on the sidewalls. The sidewall film growth eventually closes off the via before it is filled. This problem is acute in the case of multi-layer metal (MLM) designs where high aspect ratio vias are etched into a dielectric layer and metal must be deposited to fill the via. Another problem in MLM designs is that dielectric must be deposited in high aspect ratio features after each metal layer is formed and patterned before a subsequent metal layer can be formed and patterned.
Low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD) are widely used to provide conformal deposition of thin films over three dimensional features. CVD techniques are successfully used for moderate aspect ratio structures where sidewall deposition does not close off the structure before it is filled. However, most CVD systems inherently deposit on the sidewalls at the same rate as at the bottom of a trench or via, which is unacceptable for higher aspect ratio structures. CVD deposition techniques must be altered to preferentially deposit at the bottom of three-dimensional structures. Further, not all materials can be adapted to CVD deposition. In particular, few practical systems for CVD deposition of metal films exist.
Metal film deposition is widely performed by physical deposition techniques (i.e., evaporation and sputter-deposition). In the case of via filling, it is desirable to deposit films that form preferentially at the bottom of the via rather than on the sidewalls of the via. Physical deposition techniques, however, produce streams of particles that are isotropic (i.e., not highly directional). Hence, physical vapor deposition are typically limited to low aspect ratio structures.
Recently, collimated sputtering has been attempted using lattice-shaped collimators to block particles traveling towards the substrate at unacceptable angles. One such structure is shown in U.S. Pat. No. 4,717,462 issued to Homma et al. on Jan. 5, 1988. The collimators are high aspect ratio tunnels that allow only particles having acceptable trajectories to pass through. The remaining particles impact and deposit on the sidewalls of the collimator. A typical distribution of trajectories from a sputter target includes less than 50% of the particles having a trajectory of 90xc2x15xc2x0 with respect to the substrate surface.
Hence, most of the particles deposit onto the collimator rather than the substrate. This results in low deposition rates and a high degree of particulate contamination from the material deposited on the collimator sidewalls. Moreover, collimators provide limited directionality as the particles leaving the collimator still have xc2x15xc2x0 variation in their trajectories. A need exists for a high deposition rate, low particulate collimator for film deposition.
U.S. Pat. No. 4,925,542 issued to Kidd on May 15, 1990 describes a plasma deposition apparatus that uses an electrostatic field to accelerate ions from the plasma longitudinally towards a substrate. A grounded screen isolates the plasma potential from the substrate potential to allow the electrostatic field to be established. Importantly, the grounded screen was not used to collimate the particulate stream, only to electrically isolate the plasma from the substrate. Also, the Kidd deposition apparatus could only impart directional acceleration force to particles that remained ionized over the entire distance from the target to the substrate. Because ions tend to recombine with charged particles in the plasma and neutralize quickly, the electrostatic acceleration could only collimate a fraction of the sputtered particles.
Step coverage and filling of high aspect ratio structures is a continuing problem in the semiconductor industry. What is needed is an apparatus and method for filling high aspect ratio gaps and vias with collimated particulate streams.
The present invention involves an apparatus for film deposition onto a substrate from a non-collimated source of target particles. A secondary ionizer creates a secondary ionization zone between the plasma and the substrate support. The isotropically accelerated target particles are ionized as they pass through the secondary ionization zone. In one embodiment a static field generator creates a static field between the secondary ionization zone and the substrate accelerating the ionized target particles along a substantially collimated trajectory perpendicular to the substrate. Optionally, a collimator is included between the secondary ionization zone and the substrate and biased to focus and accelerate the collimated target particles.